Multilayer printed wiring boards have added valuable new dimensions to printed circuit design. The design freedom resulting from the added crossover capability creates more compact and efficient packaging of complex circuits than is possible with the conventional two-sided board. Interconnection between levels, however, has also become a more complex problem than the simple plated-through interconnections on a single two-sided printed circuit board. A persistent difficulty is the smearing of the adhesive and dielectric that occurs when the multilayer board (MLB) is drilled. Smeared adhesive and dielectric materials that coat the sidewalls of the drilled holes prevent proper electrical contact between inner conducting layers when the holes are through-plated.
This problem is addressed by U.S. Pat. No. 4,012,307. The solution proposed there is to clean the holes, before plating, using a gas plasma cleaning technique. The plasma technique is represented to be superior in several respects to the chemical etching technique used earlier to attack the same problem.
We have verified that the use of plasma etching to remove adhesive and dielectric smear from the sidewalls of the holes in multilayer printed wiring boards does indeed have merit. We have found, however, that the process taught by the patent just mentioned can be improved upon significantly. That improvement is the subject of this patent.
According to the prior art, multilayer boards having holes with sidewalls coated with unwanted dielectric material are treated in a plasma chamber. The plasma formed within the chamber etches away the dielectric material and other contaminants. However, we have found that the use of the plasma chamber, according to the teachings of the prior art, leads in many cases to incomplete or ineffective removal of the dielectric material. We have determined that the major problem with the prior art etching system is the unreliable nonuniform and inefficient penetration of the plasma to the walls within the holes.
This realization lead us to investigate a more efficient plasma configuration in which the plasma is shaped to the configuration of the boards through the use of a parallel plate electrode geometry. The advantages of this approach over the plasma chamber of the prior art (using ring electrodes mounted outside the cylindrical chamber) are the relatively short diffusion path from the electrodes to the MLB, the uniformity of the concentration of the etching gas over the surface of the MLB, and the greater control over the gas flow rate over that surface.
Experiments with the parallel plate configuration were promising and indicated that development along these lines would likely produce results superior to those obtained with the unshaped plasma of the plasma chamber. However, we were unsatisfied with our results using the parallel plate configuration. In several etching attempts, residual dielectric material still remained within the holes of the MLB after the plasma treatment.